The present invention relates to an improved process for preparing unsaturated carboxylic esters.
A number of processes have been described in the literature for the synthesis of unsaturated carboxylic esters.
The catalysts generally used are acids of the sulphuric acid, para-toluenesulphonic acid, methanesulphonic acid or similar type. The water of reaction is removed in the form of an azeotrope with the esterifying alcohol or with a solvent such as cyclohexane, toluene, etc.
The use of these catalysts results in the formation of- by-products and necessitates the use of appropriate materials which are resistant to corrosion, especially with sulphuric acid. The residual acid is then neutralized. The neutralizing operations/washings are laborious and polluting.
The use of strong cationic resins of sulphonic type which do not exhibit these disadvantages is widely described in the literature, in connection with esterification.
The American patent U.S. Pat. No. 4,833,267 describes the synthesis of C1-C2 (meth)acrylic esters in a continuous regime in a stirred reactor with the aid of a mechanical stirrer operating at a power of from 0.05 to 2 kW/m3. The cationic resin is held in suspension in the reaction medium. The drawback of this technique is the mechanical stress to which the resin is subjected, resulting in the said resin becoming fragile and breaking, with a drop in catalytic activity.
The Japanese patent application JP-A-58 192 851 describes the use of a resin-filled, jacket-heated tubular catalytic reactor topped with a distillation column for removing the water of reaction. A drawback of this technique is the prolonged contact of the water with the reactants and products, thereby bringing about the formation of heavy by-products, to the detriment of the selectivity.
The American patent U.S. Pat. No. 5,645,696 describes the use of a top-fed multistage catalyst bed for the synthesis of unsaturated carboxylic esters. The reactor is composed of from 1 to 10 resin stages and is topped by a distillation column in which the water of reaction is removed. Water is evaporated with the aid of an external exchanger. The principal drawback of this system is its laboriousness and the risks of blockage of the plates by polymers.
In accordance with the present invention, a proposal is made for an enhanced process for continuously synthesizing esters of unsaturated carboxylic acids by esterifying resins of aliphatic alcohols, which overcomes the drawbacks of the processes claimed in the American patents U.S. Pat. No. 4,833,267 (breakage of resins under the effect of mechanical stirring) and U.S. Pat. No. 5,645,696 (complexity of the reaction zone, consisting of a multistage bed, with risk of fouling by polymers) and in the Japanese application JP-A-58 192 851.
According to the present invention, by combining m a first fixed bed of resin, fed from the bottom, with a If reactor consisting of a blending tank provided with a recirculation system at the bottom through a resin cartridge, high conversion rates are obtained in combination with equally high selectivities, thereby simplifying the downstream purifying operations. Moreover, the resin charging/discharging operations are extremely simple.
The present invention accordingly provides a process for preparing an unsaturated carboxylic ester by esterifying an unsaturated carboxylic acid with an alcohol in the presence of a cationic resin catalyst, the water of reaction being removed in the form of an azeotrope with the esterifying alcohol or with a solvent, characterized in that the esterification reaction is conducted by passing the mixture of reactants in upflow mode through a bed of the said cationic resin in a recirculation loop which is combined with a stirred tank in which the reactants are blended and from which the water of reaction is removed azeotropically.
Passage over the resin takes place in the upward direction so as to keep the resin in suspension and to facilitate the thermal exchanges.
In accordance with one advantageous embodiment of the present invention, a partial reaction is conducted upstream of the stirred tank by passing the mixture of reactants in upflow mode through a second bed of the cationic resin catalyst.
The unsaturated carboxylic acid employed is advantageously acrylic or methacrylic acid. As for the alcohol employed it is, preferably, a primary or secondary aliphatic alcohol, in particular a C1-C12 alcohol, such as, in particular, n-butanol, 2-ethylhexanol and n-octanol.
The overall unsaturated carboxylic acid/alcohol molar ratio is generally between 0.6 and 1, preferably between 0.7 and 0.9.
The reaction is generally conducted in the presence of at least one polymerization inhibitor selected in particular from phenothiazine, hydroquinone, hydroquinone monomethyl ether, and sterically hindered phenols, such as 2,6-di-tert-butyl-para-cresol, at levels in the medium of between 500 and 5000 ppm. Bubbling of air is generally carried out within the blending tank in order to reinforce the action of the polymerization inhibitor or inhibitors. The latter are generally introduced with the reactants at the top of the distillation column surmounting the stirred tank.
In accordance with one particular embodiment of the process of the invention, a portion of the alcohol is introduced at the top of the distillation column surmounting the stirred tank, one or more polymerization inhibitors being advantageously combined with the alcohol introduced at the top of the column, and the mass fraction of the alcohol thus introduced at the top of the column in relation to the alcohol introduced in the tank 1 being from 20 to 60%, preferably from 30 to 50%.
The temperature at the top of the bed of resin is generally between 70 and 100xc2x0 C., more particularly between 80 and 95xc2x0 C. (above 100xc2x0 C., thermal degradation of the resins is observed); the temperature of the blending tank is generally between 100 and 110xc2x0 C., with the reaction mixture exiting the blending tank being cooled to 80-90xc2x0 C. then recirculated through the bed of resin.
The cationic resin is advantageously a strong cationic styrene/divinylbenzene resin containing sulphonic groups and having an ionic capacity of between 0.5 and 2.2 equivalents/liter. By way of examples of resins, mention may be made of that sold under the name DIAION(copyright) PK 208 by the MITSUBISHI CHEM. Company and those sold under the names XE 586, XE 386 and AMBERLIST 39 by the ROHM and HAAS Company.
Generally, the reaction mixture is circulated through the (first) bed of resin, the overall residence time on the (first) bed of resin being generally between 0.5 and 2 h.
The water generated by the esterification is distilled off in the form of an azeotrope. It is possible, as already indicated, to use an aliphatic or aromatic solvent which forms an azeotrope with water, such as cyclohexane, heptane, toluene, etc. As a general rule, however, it is the alcohol itself which plays this part.
The vapours which distil in the column contain, besides the water, the esterifying alcohol, the carboxylic ester and the carboxylic acid. A return flow of alcohol J containing from 500 to 1000 ppm of inhibitors as described above makes it possible to limit the ascent of acid at the top and the losses via the aqueous decantation phase.
The vapours condensed at the top of the column are decanted at ambient temperature:
the organic phase is returned to the distillation column, with its octene concentration being regularly lowered; and
the aqueous phase is removed.
The working pressure at the top of the distillation column surmounting the stirred tank is generally from 9.33xc3x97103 to 5.3xc3x97104 Pa (from 70 to 400 mmHg).
In the abovementioned preferred embodiment in which a first resin stage is sited upstream, the temperature at the top of the second bed is generally from 80 to 95xc2x0 C. and the reactor containing the second bed is operated at atmospheric pressure under conditions of thermodynamic equilibrium; the residence time on the resin of the second bed is generally from 20 to 60 minutes. Moreover, at the input to the second catalytic bed, the acid/alcohol molar ratio is generally top-heavy in acid.